Teratogenicity has been established for only a few drugs, but many more are still not proved to be safe for use during pregnancy. The physician should have a good reason for prescribing any drug early in pregnancy, or indeed during the last half of the menstrual cycle, when any fertile, sexually active woman might become pregnant.
Little is known about the effects of marijuana on the fetus, but major deleterious consequences have not been reported. Heroin, cocaine, and methadone, on the other hand, are associated with major problems in the neonate, especially potentially fatal withdrawal symptoms.

[...] However, the precise level of alcohol consumption during pregnancy that causes adverse fetal effects has not been established. Moreover, the chronic alcoholic may suffer from malnutrition, to the extent that the craving for alcohol exceeds the desire for food. A fetal alcohol syndrome following maternal ethanol ingestion has been described, with an incidence varying from 1 in 1500 to 1 in 600 live births, depending apparently on variations in drinking practices. The rate of reported cases identified among newborns in the United States during 1979?1992 increased approximately 4-fold. [...]

[...] Pregnant women should be encouraged to avoid alcohol intake completely during pregnancy. If this is not possible, the intake should be reduced to a minimum. OTHER MATTERS OF CONCERN DURING PREGNANCY Intercourse There has always been a suspicion that intercourse may be responsible for early abortion. Certainly, if cramps or spotting has followed coitus, it should be proscribed. There is also evidence that coitus late in pregnancy may initiate labor, perhaps because of an orgasm that causes a uterine contraction reflex. [...]